Not sure why you say optifine didn't work with the 1.7.10 version I had no issues. Once I added first person bones to my models.
Show a picture of that bone, i don't see that bone on the Sakuya model that comes with the pack.
If you look at the last bones in that model, there is itemR,itemL, Back and Camera.
And i would expect the Camera bone was the first person view bone.
Give me some time to get all the tools setup again and I'll poke around, maybe finally get to that tutorial on transferring the bones I was going to YEARS ago.
Edit: https://imgur.com/a/GL2g8Eu Dug through and found someof my old files But to find the specific bones I might have to dig through A LOT of MMD versions my current one wouldn't give me info on the bones for sakuya. This model had none of the bones needed for tools and first person till I copied them from sakuya.
I'll do some more digging and see if I get the specifics figured out.
Edit Edit: This is WITH OptiFine_1.7.10_HD_U_D8 might give the latest version a test run later as well.
Forge 1.7.10 - 10.13.4.1614 now using OptiFine_1.7.10_HD_U_E7. Can confirm the camera bone is for first-person If your getting models from DeviantArt for example they don't have the bones for all the features so you have to copy them over. I'll see about getting a tutorial up for anyone who doesn't know how or thinks it's really complicated. It really only takes a few minutes with possibly some minor tweaking afterwards to get your positioning just right.